1. Field of the Invention
This invention relates generally to downhole tools for drilling and completing subterranean wells and methods of using these tools; more particularly, this invention relates to downhole tools for selectively providing fluid communication therethrough, and methods of using those tools.
2. Description of Related Art
In many drilling, servicing, and completion applications, it becomes necessary to isolate particular zones within the well. When it is desired to completely plug a casing downhole, for example, a bridge plug may be utilized, such as those disclosed in U.S. Pat. application Ser. No. 10/658,979, entitled “Drillable Bridge Plug” by Lehr et al., incorporated by reference in its entirety herein, and assigned to the same assignee of the present application.
In some situations, it is desirable to provide a tool downhole, which allows fluid to flow in only one direction. For instance, when fracturing (“fracing”) a well, it is desirable to provide fluid communication from the formation or reservoir to surface, while not permitting fluid to flow downwardly though the tool. In these systems, a frac plug is used. When treating a multi-zone formation, a lower zone may be treated; and a frac plug may be set above the lower zone. As the frac plug allows fluid flow in one direction only (upward), frac fluid may be pumped downhole to treat a second zone, which is above the frac plug. Once the pumping of the frac fluid ceases, production from the lower and upper zone may continue concomitantly. These steps may be repeated using additional frac plugs, depending upon the number of zones to be treated.
Cement retainers also are known to operate in a similar manner, in the reverse, allowing fluid (such as a cement slurry) to be pumped downhole; however, the cement retainer operates to prevent the cement or other fluids from flowing uphole through the tool. In short, frac plugs and cement retainers are known which have a one-way valve to selectively provide fluid communication through a downhole tool. Thus, a need exists for various downhole tools adapted to control the flow of flow of cement, gases, slurries, or other fluids through the downhole tool.
One prior art system for controlling the flow of fluid through a downhole tool is exemplified by the tool having packer on a hollow mandrel, the mandrel having an inner diameter which is not uniform. As shown in FIGS. 1 and 2, a point, the diameter of the mandrel 3 narrows with sloping sides to create a ball seat 2. The ball seat 2 may be located toward the upper end of the mandrel 1 as shown in FIG. 1, or on the lower end of the mandrel 3 as shown in FIG. 2. Resting within the ball seat 2 is a ball 1. The combination of the ball 1 resting in ball seat 2 results in the mandrel 3 having an internal ball valve that controls the flow of fluid through the downhole assembly. The valve provides fluid communication in one direction, that direction depending on the orientation of the components.
In some prior art systems, a sealing ball 1 may be dropped from surface once the mandrel is set downhole. When the ball 1 reaches and rests in seat 2, the valve prevents fluid from flowing downward. In other systems, to reduce the time required for closing the valve, the ball 1 is maintained in closer proximity to the seat 2, by a biasing means such as a spring, e.g. In other prior art system, the sealing ball is maintained proximate the ball seat by a pin or cage. Until a predetermined flow rate is achieved, the ball does not seat in the ball seat; once the predetermined flow rate is established (downwardly for a frac plug; upwardly for a cement retainer), the ball 1 rests in the ball seat 2 to prevent fluid flow therethrough.
In other prior art system, the ball and ball seat are inverted from the tool shown in FIGS. 1 and 2 such that the ball and ball seat act to allow fluid, such as a cement, slurry to be pumped from surface through the downhole tool and into the wellbore, but preventing the cement from returning to surface through the downhole tool.
In some instances, once the frac plugs or cement retainers have completed their function, the frac plugs and cement retainers are destructively removed. Once removed, two-way fluid communication is allowed in the wellbore.
When it is desired to remove these ball valves, a drill or mill may be used. Components of prior art ball valves, ball and ball seats, and caged ball designs can tend to rotate with the mill or drill bit upon removal. For example, it has been discovered that when the rotating element of the removal tool, such as the mill or drill bit, encounters the ball 1, the ball 1 will being to spin or rotate along with the mill or drill bit. The ball may begin to rotate at the same speed of the mill, the ball rotating within the ball seat. Thus, the ball begins to spin within the ball seat 2 thus hampering the milling or drilling operation. When this occurs, the removal time is increased; the operator at surface may have to raise and lower the mill or drill, change the speed of rotation, etc. These actions decrease the predictability of the removal time as well as increasing the removal times, thus further increasing the cost of the removal operation. It would therefore be desirable that the downhole tool provide relatively quick and predictable times for removal. Regarding removal, it is desirable that the downhole tool be capable of being removed with a motor on coiled tubing, as opposed to requiring a drilling rig. This minimizes the expense of the removal of the downhole tool.
In some situations, the prior art gravity valves of the downhole tool may operate at a less than optimum level, depending on the downhole fluid being used. For instance, if the density of the downhole fluid is significantly lower than that of the material of the ball, the ball valves operate in a sluggish fashion, staying closed longer than desired. Alternatively, if the density of downhole fluid approaches the density of the ball, the ball may tend to “float” excessively again Thus, it is desirable that the gravity valve be weighted so that the valve operates at an optimum level closes under the force of gravity even in high specific gravity fluids.
In addition, frac plugs and cement retainers may be exposed to significant pressures downhole. Excessive pressures on the prior art ball in the ball sleeve have been known to cause the ball and seat to leak or even break under the excessive pressure. Further, partially due to the spherical nature of the contact surface of the ball with the ball seat, prior art valves may tend to leak. Thus, it would be desirable to provide a more robust, easily removable downhole tool with improved sealing function, that is capable of operating at high pressures downhole.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the issues set forth above.